Digital image watermarking apparatus and method

ABSTRACT

The present invention relates to a digital image watermarking apparatus and method. Luminance component of a digital image are transformed into coefficients of a frequency domain, and watermark is generated by encrypting a user key and generating pseudo random number from encrypted user key. Then, a replacement coefficient generated from a coefficient of a first frequency area and the watermark is replaced with a coefficient of a second frequency area so that the watermark is embedded into the digital image. The embedded watermark is extracted at an authenticating apparatus and the digital image is authenticated by finding the correlation between the extracted watermark and watermark for authentication.

CROSS-REFERENCE TO OTHER APPLICATIONS

This Application is a National Phase of International Application No.PCT/KR01/01524, filed on Sep. 10, 2001, which claims priority fromKorean Patent Application No. 2000/53755, filed on Sep. 9, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital image watermarking apparatusand method, and more specifically, to a digital image watermarkingapparatus and method which are able to extract the embedded watermark,and simultaneously not deteriorate the quality of a digital image, andwhich are able to extract the watermark without an original image byleaving a watermark in spite of print or various image alterations.

2. Description of the Related Art

As creation and distribution of-digital media, such a still image as apicture image or such a moving picture as an animation increase, thedigital media is circulated through various storage media or network,and thus illegal copy or illegal alteration is largely increased.Various devices have been developed to prevent the digital media fromillegally being altered or copied or circulated. As a device forimplementing such object, the inventor of the present invention paidattention to a digital watermarking technology.

The digital watermarking technology is to prevent copy, distribution,alteration, sale, etc. without permission of the ownership holder of thedigital data works by additionally embedding informationundistinguishable with the naked eye in the digital data works.

There are a spatial domain method, a frequency domain method, a spreadspectrum communication method, etc. in the digital watermarkingtechnology widely known up to now. These digital watermarkingtechnologies have some advantages in that it is easy to embed awatermark (spatial domain method) and it is difficult to remove awatermark (frequency domain method), etc., whereas, they have variousdisadvantages in that it is weak in an operation like filtering (spatialdomain method), an image is damaged according to the value ofcoefficient (frequency domain method), or an original image is necessaryfor extracting a watermark and an original image is largely altered byan embedding watermark (spread spectrum communication method).

That is, the conventional digital watermarking technologies as describedabove failed to meet all the requirements, in particular, i) it must bedifficult or impossible to remove the watermark embedded in a digitalimage (difficulty of access); ii) even if the digital image is printedor an image alteration for printing, for example, dithering or halftone,is made, the watermark embedded in the digital image must be robustenough to be extracted after such print or image alteration (robustnessagainst alteration): iii) when a watermark is embedded in the digitalimage, the deterioration of quality of the digital image by watermarkmust be minimized (conservation of quality of the digital image); andiv) the embedded watermark can not be known except by a copyright holderor a person whose use is allowed (hiding of the embedded information) toprevent the illegal alteration, copy and distribution of the digitalmedia thereby its range of application is very restrictive and it isdifficult to fully obtain the object described above.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide adigital image watermarking technology that meets all the requirements asdescribed above. More specifically, it is an object of the presentinvention to provide a digital image watermarking apparatus and methodin that the extraction of the embedded watermark is possible despiteprinting or various image alterations, access to the watermark becomesdifficult by embedding invisible watermark, and an original image is notnecessary in extracting watermark.

It is another object of the present invention to provide a digital imagewatermarking apparatus and method wherein the quality of the digitalimage is not largely deteriorated and it is very strong in conservingthe watermark when the digital image with watermark embedded is printedhaving high resolution or the image is altered due to dithering orhalftone, etc. since a watermark is embedded using a frequency domaincharacteristic.

It is still another object of the present invention to provide a digitalimage watermarking apparatus and method in that user key value inputtedby a user is encrypted and generated as watermark and then embedded,after extracting watermark from a digital image, by determining whetherthe watermark generated from a user key input from a user is the same asthe watermark extracted, it must grant authentication only if twowatermarks are the same as each other and thus the security of watermarkis much intensified.

It is still another object of the present invention to provide a digitalimage watermarking apparatus and method that maximizes accuracy of awatermark extraction and authentication by using the sharpening filter,the fourth moment (Kurtosis), peak value and its location thereof in thecorrelation between a user key value and watermark.

In order to achieve the object as described above, the present inventionprovides a method for embedding watermark into a digital image having aluminance component, comprising the steps of: (a) transforming saidluminance component into coefficients of a frequency domain having afirst frequency area and a second frequency area; (b) generating areplacement coefficient from a coefficient of said first frequency areaand said watermark; (c) replacing a coefficient of said second frequencyarea with said replacement coefficient, whereby said watermark isembedded into said digital image; and (d) transforming said digitalimage embedded with said watermark into an inverse frequency domain.

In order to achieve another object of the present invention, the presentinvention provides a method for authenticating the digital image,comprising the steps of: (e) transforming the luminance component of thedigital image embedded with said watermark into the coefficients of thefrequency domain; (f) generating a watermark for authentication; (g)extracting said watermark embedded in said digital image by using thecoefficients of said first and second frequency domains; and (h)authenticating said digital image by finding a correlation of saidextracted watermark and said watermark for authentication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating schematically the constitution ofa watermark-embedding device according to the present invention.

FIG. 2 is a view illustrating a discrete wavelet transform process by afilter bank used in the present invention.

FIG. 3 is a view illustrating a discrete wavelet inverse transformprocess by a filter bank used in the present invention.

FIG. 4 is a view illustrating a general distribution of coefficientsafter a discrete wavelet transform used in the present invention.

FIG. 5 is a flow chart illustrating a process which embeds a watermarkinto a digital image according to the present invention.

FIG. 6 is a block diagram illustrating schematically the constitution ofa watermark-authenticating device according to the present invention.

FIG. 7 is a flow chart illustrating a process that extracts a watermarkfrom a digital image and outputs the authentication result from thecomputation and analysis of the correlation of the extracted watermarkaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, a digital image watermarking apparatus and method accordingto the preferred embodiment of the present invention referring to thefigures attached are explained in detail.

In general, a digital image watermarking device is comprised of adigital image watermark embedding device for embedding watermark in adigital image and a digital image watermark authenticating device forauthenticating image with watermark embedded. Such devices can be usedin an independent or combination manner according to variousembodiments.

First, referring to FIG. 1 and FIG. 5, a device and method that embed awatermark in a digital image are explained together. FIG. 1 is a blockdiagram illustrating schematically the constitution of a watermarkembedding device according to the present invention and FIG. 5 is a flowchart illustrating a process which embeds a watermark in a digital imageaccording to the present invention.

A series of processes executed by a watermark embedding device 100 ofFIG. 1 corresponds to a process illustrated in FIG. 5. FIG. 1 shows theconstituents, which are included in a watermark embedding device 100 byfunction blocks, and FIG. 5 shows the proceedings executed by theconstituents of FIG. 1.

First, an original image (OI) is inputted into a grey/color imagediscriminator 110 which is included in a watermark embedding device 100(step 310). The grey/color image discriminator 110 is arranged forth todetermine whether the original image (OI) for embedding a watermark is agrey image or a color image. The grey/color image discriminator 110extracts image information data relating to input original image (OI)format, etc. and then discriminates whether it is a grey image or acolor image from the extracted image information data (step 320).

Next, the original image (OI) which experiences the discriminatingprocess (step 320) by a grey/color image discriminator 110 istransformed to a frequency domain by a frequency domain transformer 125.At this time, since a grey image has only a luminance component, in thecase the result by a grey/color image discriminator 110 is a grey image,only a frequency domain transform regarding the luminance component isexecuted and then at once a frequency transform by a frequency domaintransformer 125 is executed (step 330 a).

Differently from this, in the case the result in the discriminator 110mentioned above is a color image, first in a color model transformer120, for example, a color model like RGB mode is transformed to a colormodel of HSB (H: hue, S: saturation, B: brightness) mode, YIQ (Y:luminance, I: in-phase, Q: quadrature) mode or YCbCr (Y: luminance,Cb/Cr: chrominance) mode, and the luminance component of the transformedcolor model is extracted (step 325). The luminance component of thecolor model transformed by a color model transformer 120 is transformedto a frequency domain by a frequency domain transformer 125 (step 330b).

There are many varieties of methods to transform the original image (OI)to the frequency domain in that, representatively, FFT (Fast FourierTransform), DCT (Discrete Cosine Transform), DWT (Discrete WaveletTransform), etc. can be used.

The present invention uses the frequency domain method for embedding andextracting a watermark. The frequency domain method has advantages inthat it makes removal of watermark difficult by hiding a watermark inthe digital image and does not largely deteriorate quality of theoriginal image. In accordance with the frequency domain transformer 125,DCT coefficient, FFT coefficient or DWT coefficient (FOI) can beobtained.

Meanwhile, it is difficult to embed watermark information with a Fouriertransform coefficient because Fourier transform coefficients of thetransform domain comprise complex numbers, whereas DCT includes acharacteristic similar to Fourier transform, but coefficients of thetransform domain comprise real numbers. Hence, DCT is more advantageousand easier than Fourier transform. Hereinbelow it is explained as theembodiment that embeds watermark in the DCT coefficient or DWTcoefficient regarding the digital image. DCT is basically in a closerelation with FFT and is a transform method widely used in the standardJPEG compression, etc.

The first dimensional DCT transform is defined as in the followingequation 1a:

$\begin{matrix}{{t(k)} = {{c(k)}{\sum\limits_{n = 0}^{N - 1}\;{{s(n)}\cos\frac{{\pi\left( {{2n} + 1} \right)}k}{2N}}}}} & \left\lbrack {{Equation}\mspace{14mu} 1a} \right\rbrack\end{matrix}$

In the above equation 1a, s is an original signal value, t is atransformed signal value, N shows the length of signal, and coefficientc is as in the following equation 1b:c(o)=√{square root over (1/N)}, c(k)=√{square root over(2/N)}  [Equation 1b](just in case of 1≦k≦N−1)

On the other hand, the second dimensional DCT regarding a square matrixis defined as in the following equation 2a:

$\begin{matrix}{{t\left( {i,j} \right)} = {{c\left( {i,j} \right)}{\sum\limits_{n = 0}^{N - 1}\;{\sum\limits_{m = 0}^{N - 1}\;{{s\left( {m,n} \right)}\cos\frac{{\pi\left( {{2m} + 1} \right)}i}{2N}\cos\frac{{\pi\left( {{2n} + 1} \right)}i}{2N}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 2a} \right\rbrack\end{matrix}$

In the above equation 2a, N, s, and t have the same meaning as in thefirst dimensional DCT transform, c(i,j) is as in the following equation2b:c(o,j)=1/N, c(i,o)=1/N, c(i,j)=2/N  [Equation 2b](just in case of i≠0, j≠0)

Further, the DCT has an inverse transform and is defined as thefollowing equations 3a and 3b regarding each of the first and seconddimensions:

$\begin{matrix}{{s(n)} = {\sum\limits_{k = 0}^{N - 1}\;{{c(k)}{t(k)}\cos\frac{{\pi\left( {{2n} + 1} \right)}k}{2N}}}} & \left\lbrack {{Equation}\mspace{14mu} 3a} \right\rbrack \\{{s\left( {m,n} \right)} = {\sum\limits_{i = 0}^{N - 1}\;{\sum\limits_{j = 0}^{N - 1}\;{{c\left( {i,j} \right)}{t\left( {i,j} \right)}\cos\frac{{\pi\left( {{2m} + 1} \right)}i}{2N}\cos\frac{{\pi\left( {{2n} + 1} \right)}i}{2N}}}}} & \left\lbrack {{Equation}\mspace{14mu} 3b} \right\rbrack\end{matrix}$

Next, the DWT is reviewed. Fourier transform uses a sine function andcosine function as its basis function, whereas Wavelet transform useswavelet as its basis function, which is divided into a continuouswavelet transform and discrete wavelet transform. The continuous wavelettransform is defined as in the following equation 4.

$\begin{matrix}{{W\left( {s,\tau} \right)} = {\int_{- \infty}^{\infty}{{f(t)}{\psi\left( {s,t} \right)}\ {\mathbb{d}t}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In the above equation 4, s indicates scaling, τ indicates translation, ψ(s, t) indicates wavelet which is scaled and translated.

The scaling is related to a frequency. A low scaling, i.e., thecompressed wavelet extracts a high frequency component and a highscaling, i.e. the extended wavelet extracts a low frequency component.Usually, it is impossible to virtually realize the continuous wavelettransform since the wavelet coefficients obtained from the transform areinfinite as a function of scaling and translation.

Hence, it can be more effectively realized if the discrete wavelettransform which selects only a certain frequency domain is usedregarding the scaling and translation. However, since the discretewavelet transform also must stand a lot of computing amount to berealized by a computer, it is preferable to transform the image by usinga fast wavelet transform of a filter bank. This method uses theconventional two channels of a sub-band coding and a pyramid algorithmand has advantage that it can be easily realized only if the relationPR-QMF (Perfectly reconstruction quadrature mirror filter) betweenfilter banks for an inverse transform is carried out.

The abstract of the fast wavelet transform using filter bank is as shownin FIG. 2, FIG. 3, and FIG. 4. FIG. 2 illustrates the process of adiscrete wavelet transform by a filter bank used in the presentinvention, FIG. 3 illustrates the process of a discrete wavelet inversetransform by a filter bank used in the present invention, and FIG. 4illustrates a general distribution of coefficients after a discretewavelet transform used in the present invention.

Meanwhile, in accordance with the first embodiment of the presentinvention, in generating the watermark, a designated user key isinputted from authorized user, and the watermark is generated by theinputted user key. Should the user key be adopted as the watermark, viathe correlation between the extracted watermark and the watermarkgenerated from the user key, the authenticity or any change made will beaccurately determined. Thus, the present invention has an advantage thatstrengthens the security of the watermark.

Furthermore, in accordance with one embodiment of the present invention,after encrypting the inputted user key, a watermark to be embedded inthe digital image is generated. If the user key undergoes the encryptionprocess, not only the value of the user key will not be easilytransformed but also the user key itself can be hidden. Thus, when thewatermark generated from the encrypted user key is embedded in thedigital image. The present invention has an advantage as the user keyitself will not afterwards raise any concern that the user key itselfwill be revealed when extracting the watermark from the digital image.

The encryption may employ the conventional encryption methods, forexample, data encryption standard (DES) method, RSA method, ECC method,etc. According to one embodiment of the present invention, asillustrated in FIG. 1, a PN-code method which outputs in pseudo randomnumber the result of the coded data that will be encrypted is employed.

In order to generate the watermark (W), as illustrated in FIGS. 1 & 5,should the user key be inputted in the watermark generator 130 (Step341), the user key encryptor 131 and pseudo random number generator 132included in the watermark generator 130 encrypt the user key using theinputted user key, generate pseudo random number (Steps 343 & 345), andgenerate watermark (W) which will be embedded in the frequency domaintransform coefficient (FOI) of the original image (OI) (Step 340).

The watermark (W) generated from the user key is embedded in the digitalimage via a process such as the following equation 5 (Step 350 a or Step350 b).FOI′=ƒx(|FOI|)×W  [Equation 5]

wherein, ƒx is a characteristic function for controlling the intensityof the watermark which will be embedded in the original image. ƒx may bevariously set up which can be a function either performing an modulooperation of portion of the frequency domain transform coefficient (FOI)of the original image (OI) or rendering statistical values of themultiple transforming coefficients such as the mean, standard deviation,and variance etc.

FOI′ represents the result of multiplying watermark (W) to a valueapplying characteristic function ƒx to the frequency domain transformingcoefficient (FOI) of the original image (OI). The transform coefficient(FOI) in the above equation 5 is a value of the frequency domain of thedigital image, therefore is in a two dimensional arrangement format. Thewatermark (W) has one dimensional sequence format so that thetransforming coefficient (FOI) is transformed into a sequence format orthe watermark is transformed into a two dimensional block to perform thecalculation. For performing the calculation, there is a method whereinper a frequency coefficient of the image, a watermark is matched theretoto calculate or a method wherein the statistical characteristics ofvarious frequency coefficients of the image are selected to performcalculation with a watermark.

To explain the process of embedding the watermark in more detail inreference to the equation 5, the watermark embedder (140) executes aprocess replacing a transform coefficient of another designatedparticular frequency area with the result (FOI′, hereinafter refer to as“replacement coefficient”) of multiplying the value obtained fromapplying characteristic function ƒx to the absolute value of thetransform coefficient of the designated particular frequency area(hereinafter, refer to as “embedding coefficient”) by watermark.

For example, when the digital image is divided into low frequency,middle frequency and high frequency areas, the watermark is embeddedinto the middle or high frequency areas containing relatively lessdigital image information. The watermark embedding is completed byreplacing the transforming coefficient of the existing middle or highfrequency areas with the result (replacement coefficient) of multiplyingthe result from applying characteristic function ƒx to the transformcoefficient (embedding coefficient) of the low frequency area bywatermark.

According to such method, even though there is a compressing processabandoning the high frequency portion or a transformation, such asblurring, the watermark is multiplied by the coefficient of the lowfrequency area and replaced to the high frequency area, after thetransformation, relatively large portion of the high frequency arearemains. Thus, the present invention has an advantage of being robust toagainst the transformation.

The present specification sets forth the replacement of the watermarkmultiplied by the result obtained from applying the characteristicfunction ƒx to the absolute value of the coefficient of the lowfrequency area between to the areas the middle frequency and highfrequency as an example. However, the watermark can be embedded byselecting a frequency area with range that does not degrade the qualityof the digital image. For instance, as it is possible to replace thewatermark to the high frequency area after being multiplied by theresult from applying the characteristic function to the absolute valueof the transforming coefficient of the high frequency area, a methodembedding the watermark in the digital image transformed to thefrequency area is not limited thereto, but is also applicable to severalfrequency areas.

Also, in embodiments of the present invention, the overallcharacteristics of the image are not easily changed from the print andadopt the characteristics of the frequency domain, which does not changethe overall characteristics of the image by compensating the coefficientof the other frequency area with the arbitrary coefficient of thefrequency area, making it sturdy at printing. Thus, disappearance of thewatermark at printing, etc. will be prevented. Furthermore, according tothe watermark embedding method using the aforementioned characteristicsof the frequency domain, because the overall characteristics of thedigital image will not change even at the embedding of the watermark,i.e. the quality of the original image is not degraded, and it ispossible to enjoy the quality of the original image even after beingprinted.

In equation 5, the intensity of the robustness of the watermark (W) whenthe watermark (W) is embedded in the original image (01) is controlledby function ƒx. Any value can become said characteristic function valuesince the value can be set up by user as he pleases, and particularly,an appropriate function that does not degrade the quality of the digitalimage from embedding the watermark (W) can be determined. In otherwords, intensity of the robustness of the watermark is indirectlyrelated with quality of the digital image or vice versa. Therefore, itis desirable to make a determination of an environment in which thewatermark is used and a maintenance of the quality of the digital image.

Now, in order to acquire digital image embedded with watermark, aninverse frequency domain transformer (150) restores the transformeddigital image. (Step 360 a or Step 360 b) When the aforementionedfrequency domain transformer 125 employs discrete cosine transform (DCT)or discrete wavelet transform (DWT) or fast fourier transform (FFT), theinverse frequency domain transformer 150 employs inverse discrete cosinetransform (IDCT) or inverse discrete wavelet transform (IDWT) or inversefast fourier transform (IFFT). At this point, when the original image(OI) is in color image, the color model of HSB mode, YIQ mode or YCbCrmode is restored by inverse color model transformer 155 to the originalcolor model from employing the remaining constituent which did notundergo the frequency domain transformation and the luminanceconstituent wherein the inverse frequency domain transformation isconducted (Step 370). Finally, by acquiring the digital image (WI)embedded with the watermark (Step 380), the embedding process of thewatermark to the digital image is completed.

Referring to FIG. 6 and FIG. 7, the authentication result output deviceand the method of extracting watermark from the digital image embeddedwith watermark and via correlation between the extracted watermark andthe watermark generated from the user key are explained below.

FIG. 6 is a block diagram illustrating schematically the constitution ofthe watermark authenticating device according to the present invention,and FIG. 7 is a flow chart illustrating the process of extractingwatermark from the digital image according to the present invention andoutputting the authentication result from the computation and analysisof the correlation to the extracted watermark.

A series of process performed by the watermark authenticating device 200of FIG. 6 corresponds to a process illustrated in FIG. 7. FIG. 6 depictsthe constituent elements included in the watermark extraction andauthentication device 200 in blocks according to their functions, andFIG. 7 depicts the extraction and authentication processes performed bythe constituent elements of FIG. 6.

First, the digital image (WI) embedded with watermark is inputted intothe grey/color image discriminator 210 of the watermark extraction andauthentication device 200 (Step 410). The grey/color image discriminator210, color model transformer 220, and frequency domain transformer 225of FIG. 6 are identical to the grey/color image discriminator 110, colormodel transformer 120, and frequency domain transformer 125 of FIG. 1 inaspect of their operation and function.

In other words, the image (WI) embedded in the watermark is identifiedwhether it is in grey image or in color image (Step 420), and should theimage embedded in the watermark be in grey image, the frequency domaintransform with regard to luminance constituent is conducted by frequencydomain transformer 225 (Step 430 a). If the image (WI) embedded in thewatermark is in color image, first, the image is transformed to colormodel such as HSB mode, YIQ mode or YCbCr mode, etc. by color modeltransformer 220 (Step 425). Then, by the frequency domain transformer225, the frequency domain transform such as discrete cosine transform(DCT) or discrete wavelet transform (DWT) or fast fourier transform(FFT) regarding luminance constituent is conducted (Step 430 b).

As illustrated in FIG. 6 and FIG. 7, should the user key be inputted inthe watermark generator 230 (Step 441), the user key is encrypted by theuser key encryptor 231 (Step 443) in the identical method as thewatermark embedding process of FIG. 5, and watermark (W) according toPN-code method is generated by pseudo random number generator 232 (Step445). The watermark generator 230 and the watermark generating process440 of FIG. 6 and FIG. 7 are identical to the watermark generator 130and the watermark generating process 340 of FIG. 1 and FIG. 5.

Next, the correlator 240 performs the inverse process of watermarkembedding to extract a watermark (WE) embedded in the digital image fromthe frequency domain transform coefficient (FWI) of the digital imageembedded with the watermark obtained from the result of the frequencydomain transform, and computes the correlation between the twowatermarks (W and WE) (Step 450) During watermark extraction, theprocess multiplying the result of applying the characteristic functionto the coefficient of the designated particular frequency area by thewatermark and embedding into a coefficient of the other designatedparticular frequency area coefficient is inversely processed. From thedigital image embedded with watermark, the replacement coefficient(FOI′) embedded in the aforementioned particular frequency area and theembedding coefficient (FOI) used in the replacement coefficient (FOI′)calculation are obtained, and using these embedding coefficient andreplacement coefficient, the watermark is restored. The correlationbetween the restored watermark (WE) and the watermark (W) generated by amethod used in the watermark embedding is obtained so that it ispossible to determine whether the designated watermark is in the digitalimage.

Though it has not been illustrated in the drawings, other than themethod generating the watermark from the user key, for example, there isan embodiment embedding and extracting any one of the various watermarksstored in advance in the designated storing medium. For example, whenthe user key is not inputted, it is possible to determine whether thepredetermined watermark is embedded by comparing the watermark obtainedfrom the embedding coefficient and the replacement coefficient, forexample, with the various watermarks stored in advance.

In any case, the watermark can be directly extracted from coefficientvalues of the frequency domain of the image embedded with watermark,therefore the original image is not required.

Nevertheless, when the watermark has a sequence with arbitrary values,in order to restore the watermark, the embedding coefficient and thereplacement coefficient values should be used. For example, if thewatermark contains binary sequence of only 1 and −1, the value of thewatermark can be restored only by the signs of the replacementcoefficient under the assumption that the value of characteristicfunction ƒx is always positive.

The correlator 240 obtains the correlation between the watermark (WE)extracted as above and the watermark (W) generated by employing the keyinputted by the user and the correlation values thereof to confirm thewatermark embedding designated by the user. Thus, it is possible to knowby watermark extraction and authentication device 200 whether thedigital image under inspection is authentic or altered.

The method to obtain the correlation between the two watermarks (WE andW) of the correlator 240 and the correlation values thereof is explainedbelow.

The correlation between the two watermarks (WE and W) can be obtainedfrom the following equation 6:

$\begin{matrix}{{{Corr}\left( {{WE},W} \right)} = {{\sum\limits_{k = 0}^{N - 1}\;{{WE} \cdot W}} \equiv {{real}\left( {{IFFT}\left( {{{FFT}({WE})} \times \left( {\overset{\_}{{FFT}(W)} \times {{FFT}(W)}} \right)} \right)} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

wherein, FFT(W) is a conjugate complex of FFT(W), and IFFT(W) representsan inverse FFT(W).

The result of the correlation obtained from the above MathematicalFormula 6 is not a certain value, but is in a sequence form (X₁, - - - ,X_(N)). Accordingly, the maximum peak value M and its position P areobtained in comparison of the obtained plurality numbers of values(X₁˜X_(N)).

Next, from the result of the correlation obtained from the followingequation 6, the sharpness, i.e. the forth moment Kurtosis (K) isobtained from the equation 7.

$\begin{matrix}{{{Kurt}\left( {x_{1},\ldots\;,x_{n}} \right)} = {\left\{ {\frac{1}{N}{\sum\limits_{j = 1}^{N}\;\left\lbrack \frac{x_{j} - \overset{\_}{x}}{\sigma} \right\rbrack^{4}}} \right\} - 3}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

wherein, X₁, - - - , X_(N) is a sequence obtained from the aboveexplained equation 6, and they are the resulting values of thecorrelation between two watermarks (W and WE). χ is the average ofX₁, - - - , X_(N) and σ represents the standard deviation.

When analyzing the correlation values K, M and P obtained as above (Step460), it is possible to determine whether the watermark (W) generated bythe designated user key agrees to the watermark (WE) extracted. In moredetail, when the correlation value K, i.e. Kurtosis is less than thepredetermined threshold, the two watermarks (W and WE) do not agree toeach other, and also when the maximum peak value (M) is less than thepredetermined threshold, it represents that the two watermarks (W andWE) do not agree to each other. Furthermore, only when the position (P)of the maximum peak value of the two watermarks (W and WE) is alwaysidentical, it means that the two watermarks (W and WE) agree to eachother. Moreover, PN-code generated by the watermark generator (230) has‘0’correlation degree with PN-code excluding the signal of its own.Thus, when the correlation degree by PN-code is also the predeterminedcritical value or more, it means that the two watermarks (W and WE) aresame each other. Accordingly, when the digital image satisfies all therequirements of the above-mentioned correlation values, it means thatthe two watermarks (W and WE) agree with each other so that theauthentication of the digital image can be confirmed. For instance, aperson in possession of a certificate printed with a digital imagewatermarked in accordance with the result analyzing the requirements ofthe correlation values such as Step 370 of FIG. 7 is eitherauthenticated or for example, is permitted his pass (Step 480 a), or hisauthentication is rejection by, for example, refusing his pass or givinga warning (Step 480 b).

In order to evaluate the digital watermarking device and methodaccording to the present invention, an experiment extracting thewatermark after embedding the watermark with the digital image of size256×256 and then inflicting transformation such as dithering, halftone,and blurring etc. is conducted. With regard to this experiment, theextraction result of approximately 99.92% is obtained. The experiment iscontinuously conducted to extract watermark after the image printed froma printer in a resolution of 300 dpi is generated again into the digitalimage using a scanner and a digital camera. This experiment showed ahigh extraction rate of more than 95%. Hence, it has been proved thatthe present invention has a watermarking technique powerful against thetransformation resulting from being printed.

As described above, according to the digital image watermarkingapparatus and method of the present invention, the principle of thepresent invention using the characteristic of the frequency domainprovides such advantages that the quality of original image isguaranteed against embedment and extraction of watermark, and thewatermark that is resistant to printing or image transform can beextracted without using the original image. Also, the security andaccuracy of watermark are significantly strengthened by embeddingwatermark generated from a user key, authenticating a digital image onlywhen watermark generated from the user key inputted by a user isconsistent with the watermark as extracted when extracting watermark,and using the correlation value such as the 4th moment (Kurtosis).

The apparatus and method of the present invention, being resistant tothe printing and providing the enhanced security, are applicable tovarious applied examples requiring a copyright or authentication asprinted matters and confirmation of personal information. That is, it isprovided an information security device and method thereof aspreconditions for activating electronic commerce, an electroniccertificate of authentication, an electronic identification card, anelectronic registration, an internet coupon, electronic cash, anidentification card, a school record and the like on large-scale networksuch as internet. Also, since it is possible to prevent illegaltransformation or use such as forgery and alteration of informationneeded for secret maintenance, the present invention can be widelyapplied to the intellectual property protection industry field ofcopyrighted digital image materials as well as protection of personalinformation.

Although the preferred embodiments of the digital image watermarkingapparatus and method according to the present invention are disclosedfor illustrative purposes, it will be obvious to those skilled in theart that such embodiments are merely for illustrations but are notrestricted to the illustrations themselves, and can be variouslychanged, transformed and replaced within the scope of the technical ideaof the present invention. Further, it should be understood that thetechnical idea of the present invention is not restricted to theaforementioned embodiments but to only the claims attached hereto andtheir equivalent technical principle.

1. A method for embedding watermark into a digital image having aluminance component, comprising: (a) transforming said luminancecomponent into coefficients of a frequency domain having a firstfrequency area and a second frequency area; (b) generating a replacementcoefficient from a coefficient of said first frequency area and saidwatermark; (c) replacing a coefficient of said second frequency areawith said replacement coefficient, whereby said watermark is embeddedinto said digital image; (d) transforming said digital image embeddedwith said watermark into an inverse frequency domain; (e) transformingthe luminance component of the digital image embedded with saidwatermark into the coefficients of the frequency domain; (f) generatinga watermark for authentication; (g) extracting said watermark embeddedin said digital image by using the coefficients of said first and secondfrequency domains; and (h) authenticating said digital image by findinga correlation of said extracted watermark and said watermark forauthentication, wherein (h) comprises: (h1) finding the maximum peakvalue and the position of said maximum peak value from the correlationbetween said extracted watermark and said watermark for authentication;(h2) finding the 4th moment from said correlation; (h3) judging thatsaid extracted watermark is same with said watermark for authenticationin case where said 4th moment is over a predetermined critical value,the maximum peak value of said correlation is over a predeterminedcritical value, and the position of the maximum peak value of theextracted watermark and the watermark for authentication are the same.2. The method according to claim 1, wherein said digital image is acolor image and said method further comprises: before (a), (a′)transforming said digital image into a color model including theluminance component and extracting the luminance component of saidtransformed color model; and after (d), (d′) performing a color modelinverse transform of said digital image as transformed into said inversefrequency domain by using the luminance component for which said inversefrequency domain transform is performed, and the remaining componentswith the exception of said luminance component for which said frequencydomain transform is not performed.
 3. The method according to claim 2,wherein said color model including said luminance component is one ofHSB mode, YIQ mode and YCbCr mode.
 4. The method according to claim 1,wherein said replacement coefficient is generated in such manner thatsaid watermark is multiplied by the operation result obtained byapplying a predetermined characteristic function to an absolute value ofthe coefficient of said first frequency area.
 5. The method according toclaim 4, wherein said first frequency area is a low frequency area andsaid second frequency area is between a middle frequency area and a highfrequency area.
 6. The method according to claim 4, wherein saidcharacteristic function is the modulo operation result of parts of saidfrequency domain transform coefficients, or a function represents thestatistical characteristic of said frequency domain transformcoefficients.
 7. The method according to claim 1, wherein (a) uses oneof among FFT, DCT, and DWT.
 8. The method according to claim 1, whereinsaid watermark is generated by encrypting a user key and generatingpseudo random number from said encrypted user key.
 9. The methodaccording to claim 8, wherein said encryption is executed by PN-codemethod.
 10. The method according to claim 1, wherein said watermark forauthentication is generated by encrypting a user key and generatingpseudo random number from said encrypted user key.
 11. The methodaccording to claim 1, wherein (h) comprises: if a user key is encoded inPN-code method during the embedding process of said watermark, judgingthe consistency of said extracted watermark and said watermark forauthentication when the correlation by said PN-code method is over thepredetermined critical value.
 12. An apparatus for embedding a watermarkinto a digital image having a luminance component, comprising: afrequency domain transformer for transforming said luminance componentinto coefficients of a frequency domain having a first frequency areaand a second frequency area; a watermark generator for generating saidwatermark; a replacement coefficient generator for generating areplacement coefficient from a coefficient of said first frequency areaand said watermark; a replacing device for replacing a coefficient ofsaid second frequency area with said replacement coefficient, wherebysaid watermark is embedded into said digital image; an inverse frequencydomain transformer for transforming said digital image embedded withsaid watermark into an inverse frequency domain; a frequency domaintransformer for transforming the luminance component of the digitalimage embedded with said watermark into coefficients of the frequencydomain; a watermark generator for generating watermark forauthentication; a watermark extractor for extracting said watermarkembedded into said digital image by using the coefficients of said firstand second frequency domains; and a correlation computation device forauthenticating said digital image by finding a correlation between saidextracted watermark and said watermark for authentication, wherein thecorrelation computation device finds the maximum peak value and theposition of said maximum peak value from the correlation between saidextracted watermark and said watermark for authentication, finds the 4thmoment from said correlation, and judges that said extracted watermarkis same with said watermark for authentication in case where said 4thmoment is over a predetermined critical value, the maximum peak value ofsaid correlation is over a predetermined critical value, and theposition of the maximum peak value of the extracted watermark and thewatermark for authentication are the same.
 13. The apparatus accordingto claim 12, further comprising: a color discriminator fordiscriminating whether said digital image is a grey image or a colorimage; in case where said digital image is the color image, theapparatus further comprising: a color model transformer for transformingsaid digital image to a color model including the luminance componentand extracting the luminance component of said transformed color modelto provide said frequency domain transform means; and an inverse colormodel transformer for performing a color model inverse transform of saiddigital image transformed from said inverse frequency domain transformmeans, by using the luminance components for which said inversefrequency domain transform is performed, and the remaining componentswith the exception of said luminance components for which said frequencydomain transform is not conformed.
 14. The apparatus according to claim12, wherein said watermark generator comprises: an encrypting device forencrypting a user key; and a pseudo random number generator forgenerating a pseudo random number from said encrypted user key.